Unstable shoulder joint
Within rotator cuff (RC) we have seen how the joint acts to hold the humeral head on the surface of the glenoid by the development of concavity compression: this is the effect of the force generated by the RC on the humeral head acting on the surface area of the glenoid - the joint reaction force. So, the RC acts on the humeral head holding on the surface area of contact (SAC) or surface arc of contact of the glenoid.
The structures of the labrum and capsule attached around the margins of the glenoid are packed with mechanical receptors which sense the amplitude, direction, and speed of movement: these give feedback through the nervous system (NS) to the RC to “tell” it when and how to act to control motion. This mechanism can be called a capsulo-labral propriocetive mechanism (CLPM). So we can develop an equation which describes ‘stability’ in the shoulder:
Stability (ie a normal joint reaction force) = RC + SAC + CLPM + NS. Everyone has a rotator cuff, the SAC can vary between individuals, there is up to a 30% chance that parts of the CLPM are not developed in any individual, and the nervous system can be relatively readily upset: it follows that the function of the ever-present RC is almost certainly the most important factor in achieving shoulder stability. It follows from this that RC dysfunction is a potent cause of shoulder instability. This is true: a tear of the SSP can allow displacement of the shoulder in an upward direction (see above).
Importantly, simply saying the shoulder is “unstable” is not enough. This is not a diagnosis in itself - we have to find out what is wrong with the structure or control of the shoulder. This is then the diagnosis, and we can then prescribe treatment (in most cases).
The shoulder can displace in any direction: if this is accompanied by pain, a click, a sense of abnormal motion or apprehension, or sufficient displacement to permit the humeral head to lock out of place, we call this instability. That is, “instability” is the sense of symptomatic abnormal motion.
Instability can be caused by problems of the structure of the shoulder: trauma can lead to detachment or stretching of the capsule, labrum, or RC of the shoulder; deficiency (microtrauma or true collagen alterations) of the structural fibres of the capsule can lead to atraumatic instability. Instability can also be caused by non-structural mechanisms in which perturbation of the neurological control systems allow abnormal movement in and around the shoulder. Expert physiotherapists can pick up these perturbations very well.
Similar considerations apply to the SCJ and to the ACJ. Trauma to either can cause instability through structural damage, but in the case of the SCJ, slackness of the capsular tissues and the shallow shape of the joint contact surfaces added to aberrant neuromuscular control makes the likelihood of instability greater.
Instability of the ACJ most commonly occurs through trauma: falling at speed from a height, for instance from a bicycle, or being crushed under a scrum, account for most ACJ injuries. Here the restraining ACJ ligaments are torn and the scapula loses its suspension from the end of the clavicle in varying degrees of separation. While the obvious deformity is of the ACJ the essence of this injury is the mal-effect on the position and control of scapular movement.
The elbow is relatively easily dislocated when the arm is put out to break a fall, and the hand is planted on the ground while the body twists over it. The radial head dislocates by rotating backwards taking the ulna with it. The outcome for elbow dislocation is good: in contrast to the shoulder it is thought that single dislocations of the elbow heal well and the risk of further dislocation is very small.
First time dislocation
Depending on the direction of dislocation tissues are torn by varying degrees, and acute inflammation sets in: this is essential for the healing cascade to begin. Swedish studies (Hovelius, 1994) on the long-term outcome of first time dislocations suggest that the outlook for shoulders is good with no operation - only about 20% of people with a first-time dislocation eventually had an operation for recurrent instability and most went back to their work and sport. This is hotly debated however. A current opinion (Wallace, Imhoff, Tibone) is that top sportsmen might benefit from repair of the damage done by dislocation. Repair is often performed by arthroscopic surgery, with success rates (ie no further dislocations) of between 85 and 90% achieved.
When the shoulder (or elbow) becomes recurrently unstable then repair of the relevant capsule and specific ligaments is possible, sometimes requiring grafts or substitutes to rebuild particularly damaged bone or ligament tissues. With increasing complexity of repair or reconstruction the risk of failure ie recurrent instability increases - the success of these operations is about 75 - 80%.
When a joint is dislocated and cannot be put back into the correct position it is said to be a persistent instability (also called a locked or inveterated dislocation). This is most common in the condition of posterior (backward) dislocation in younger people, and anterior (forward) in older people (especially when there is additional RC tearing). The risk of nerve damage also increases if the joint is locked out. The nerves most at risk are the axillary nerve (which powers the deltoid muscle), the musculocutaneous nerve (which powers the biceps muscle and some skin sensation along the outer border of the forearm), and the suprascapular nerve (which powers the SSP and ISP parts of the RC, and the sensation to the upper half of the shoulder joint cavity).
Related treatment information: